Process for lowering molecular weight of liquid lignin

ABSTRACT

Processes and systems for lowering molecular weight of lignin generally includes first isolating a dense liquid lignin phase from black liquor and subjecting the dense liquid lignin phase to a temperature and pressure for a period of time to effect an average molecular weight distribution of the lignin. Solid lignin produced with the lowered molecular weight is then isolated. The systems and processes may further include an oxidation unit for oxidizing the black liquor and intermediate streams to remove or mitigate malodorous or toxic emissions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The contents of Provisional patent Application U.S. Ser. No. 62/016,833 filed Jun. 25, 2015, on which the present application is based and benefits claimed under 35 U.S.C. §119(e), is herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to a process for lowering the molecular weight of liquid lignin.

Lignin, a component of wood, is the second most abundant polymer in the world behind cellulose. With its high energy density and variety of functional groups and structure, lignin is an efficient biofuel source or green-chemical precursor. One use for lignin is to burn the solid lignin as a fuel, to or use the lignin as a binder for energy pellets. Currently wood pellets are burned, but the ash content and lower energy density limit their use as a fuel. Lignin pellets have approximately the same energy content as coal, about 12,000 Btu/lb., which is about 50% higher energy per mass of low-moisture wood pellets having about 8,000 Btu/lb. Lignin pellets may be used alone or blended directly with the coal feed with the only additional capital being the separate storage and feeding equipment for the pellets. Also lignin has demonstrated potential as an improved binder for wood or grass pellets, decreasing the dust levels generated in processing of the pellets, improving the water resistance of pellets which is important for outside storage of pellets, and increasing the energy density of the pellets.

Lignin also has great potential as a chemical feedstock for adhesives, plastics, coatings, fibers, and carbon fibers. Lignin is the most abundant source of aromatic chemistry found in nature, and as such, is a valuable potential source of benzene-, toluene-, and xylene-type chemistry. In the non-fuel applications, lignin molecular weight (MW) can impact end use properties. Lower molecular weight can, for example, lower lignin viscosity thereby promoting flow, spreading or wetting of a binder, coating or composite component, while higher molecular weight enhances other properties, like glass transition temperature and rigidity. An important application attribute in composites is the ability of the resin to flow and penetrate the fibrous matrix, completely saturating the interstitial volume with resin to minimize “dry spots” which severely limit strength properties of the final composite. Generally lignin is incorporated into a resin to displace a fraction of the petrochemical polymer used normally in the resin, and high molecular weight lignin can increase viscosity and limit flow of the resin. Decreasing molecular weight of lignin can significantly reduce its viscosity when included as a component in the resin. Thus, there is interest in developing methods to make lignins of controlled molecular weight. To understand the methods used to control molecular weight, it is useful to begin by explaining why and how lignin is prepared in the papermaking industry.

In the kraft papermaking process, wood chips are cooked in a strongly basic aqueous solution, typically an aqueous sodium hydroxide (NaOH) solution, and which may also contain sodium hydrosulfide (NaSH) to cleave and dissolve lignin and hemicellulose and leave the cellulose fibers that are subsequently filtered, washed, and formed into paper. The severity of pulping is the first determinant of molecular weight in the lignin produced. In general, the more severe the pulping, the lower is the molecular weight of the resulting lignin. The liquid containing soluble lignin, hemicellulose and spent pulping chemicals is called black liquor.

Many pulp mills today could increase their production of pulp and paper and hence their profitability were they not limited by capacity of the recovery boiler used to burn black liquor to recover heat and the valuable pulping chemicals (NaOH and NaSH). Removing a fraction (up to 30%) of the lignin from black liquor allows mills that have reached the maximum throughput of their recovery boilers to increase production by approximately the same fraction of lignin removed. A general rule-of-thumb is that 60 units of lignin is dissolved in black liquor for each 100 units of pulp fiber generated. For example, in a large mill making a million tons/yr. of pulp, the lignin dissolved in the black liquor would be about 600,000 tons/yr. Recovering 30% of the lignin from black liquor could produce 180,000 tons of lignin per year. Papermaking facilities generally have power boilers that are designed to burn residual wood (bark, limbs) from forest logging operations. Lignin is very similar to coal and can be burned in a utility company's power boilers generating electricity that are designed to burn coal but not residual wood. If a papermaking facility makes one unit of lignin, then replaces that lignin energy value is replaced in their operations by burning residual wood, then uses that lignin to displace coal at a utility company, green-house gases are reduced by an overall 2.5 units. In this manner, a large mill recovering 180,000 ton/yr. lignin could reduce green-house gases by 450,000 ton/yr. if that mill replaced the energy lost by burning residual wood and that lignin was used to displace coal. Typically, the black liquor that is fed to a separate lignin recovery process is removed midway in the evaporator train, is preferably at a solids content of 30% to 45% and has a temperature of about 80° C. to about 120° C. It should be understood that the solids content of the black liquor serving as feed for a lignin-recovery process ranges from about 20% to about 60%, but more normally is from 30% to 50%.

Lignin may be recovered from papermaking black liquor by several processes. One such process makes powdered lignin containing high-salt content (about 4% ash), which creates issues with high ash within utility power boilers. Also this method cools the black liquor and dilutes the black liquor that is returned to the host paper mills, which creates a high energy penalty.

A second process is described by U.S. Pat. Nos. 8,172,981 and 8,486,224. In this process, the black liquor is first cooled and contacted with carbon dioxide to reduce the pH from 13-14 to 9-10. As a result, lignin becomes insoluble in the black liquor and precipitates as solid particles that are filtered to remove them from the residual black liquor which is returned to the host mill. The lignin filter cake is then re-slurried, mixed with an acid to reduce the dispersion's pH from 9-10 to 2-3. At this lower pH, the cations (primarily sodium) are displaced from the phenoxy- and carboxylic-groups on the lignin polymer. The lignin dispersion is subjected to a second filtration, and the cake is washed in-situ within the filter with water to reduce the ash content of the final product lignin. This low-salt lignin (about 1%) can be used as a fuel.

The third process is similar to the second process since the black liquor is first cooled then reacted with carbon dioxide to lower the pH and precipitate the lignin as solid particles, which are then filtered to separate the lignin from the residual black liquor. The major difference is that this third process does not re-slurry the lignin after the carbonation. Instead, sulfuric acid is pumped through the filter cake to displace the sodium cations from the lignin phenoxy- and carboxylic-groups, and the cake is subsequently washed with water to remove the salt. Also, this process uses oxidation of the black liquor to improve the filtration properties. This process has similar disadvantages with respect to energy penalty to the host mill as the other two processes, in that the black liquor is cooled to recover the lignin and that a significant amount of water is used.

US Pat. Pub. No. 2011/0294991, incorporated herein by reference in its entirety, describes a fourth process for removing lignin from black liquor that includes, similar to the other processes removing lignin from papermaking black liquor, lowering system pH with carbon dioxide from the initial pH 13-14 down to pH 9-10. This process maintains the high process temperature from the host papermaking facility, separating lignin from the black liquor as a true liquid phase which is dense and separates from the residual black liquor by gravity. The heat of reaction of carbon dioxide is preserved so that residual black liquor, from which the lignin has been removed, returns to the host mill at a higher temperature than the black liquor fed. The lignin is concentrated in the liquid-lignin phase. The liquid-lignin pH is further lowered to pH 2-3 so that sodium cations are displaced from the carboxylic- and phenoxy-group functionality of the lignin polymer. This process is continuous, thus smaller equipment can be used which requires lower capital cost relative to the two competitive processes.

At least two methods are being developed to remove lignin from wood chips, which allows stand-alone operation separate from a host Kraft mill. The first process uses ethanol, co-solvents and catalysts at elevated temperature and pressure to separate the lignin and hemicellulose as a liquid phase from the solid cellulose. The lignin produced has extremely low-salt levels, less than 0.1%. This process produces two separate lignin product streams, one having a relatively low molecular weight compared to the other. Also, unlike the previously-discussed processes in which the lignin molecular weight is set by the upstream pulping process, the molecular weight of the lignin can be adjusted via adjusting the time, temperature, solvent concentrations and catalyst concentrations within the process. However the capital and operating costs are extremely high, such that the lignin cannot be considered as a fuel to replace coal.

A second process is described in US Pat. Pub. No. 2013/0239954. This process uses near-critical water and carbon dioxide to convert cellulose in biomass to sugars, leaving lignin as a residual stream. This process is reported to produce sugars at competitive value to corn, so the resultant lignin likely could be used as a bio-fuel.

Biodegradation of lignin, sometimes termed enzymatic degradation of lignin, is a key aspect of wood rotting. Research in this area continues, with added impetus provided by the need to develop better cellulosic ethanol technology. Because enzymes are thermally fragile, temperatures must be kept low, so rates are generally slow, the enzymes are costly, carbon dioxide byproduct formation costs yield, and the lignin-derived products are contaminated with residual enzyme and its degradation products. Thus, there is a continuing search for more effective and less costly methods for controlling lignin molecular weight.

Methods for lowering the molecular weight of isolated lignin have been disclosed U.S. Pat, Pub. 2008/0050792 and U.S. Pat. No. 7,964,761B2. This concept requires isolation of solid lignin as feedstock for the molecular weight lowering process. As a result, its use suffers all the drawbacks noted above for processes that isolate lignin. Thus, there is a continuing search for ways to simplify the process and make it more economical.

U.S. Pat. Pub. No. 2013/0131326 discloses a process for reducing the molecular weight of lignin by increasing the temperature of black liquor to 170° C. to 190° C. for a period of time of about 1 to about 60 minutes followed by lignin precipitation. This temperature is higher than the normal pulping temperature of about 150° C. that is generally considered to determine the molecular weight of the lignin in the untreated black liquor. One of the problems with reducing molecular weight in this manner is that the process is energy intensive since all of the black liquor is being treated to effect molecular weight reduction. As a result, the equipment is relatively large and requires pressurization to prevent boiling, and thus relatively expensive. Moreover, because all of the black liquor is exposed to the elevated temperatures to effect molecular weight reduction, the non-lignin components within the black liquor are also exposed to the higher temperatures, which can degrade and significantly decrease the value for some of these components.

SUMMARY OF THE INVENTION

Disclosed herein are processes and systems for lowering molecular weight of lignin. In one embodiment, a process for reducing molecular weight of lignin comprises: carbonating a black liquor stream and adjusting the pH above 10; recovering a dense liquid-lignin phase, wherein lignin within the dense liquid-lignin phase has a first average molecular weight; and exposing the dense liquid lignin to heat and pressure for a period of time to reduce an average molecular weight of the lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight.

In another embodiment, the process for reducing the molecular weight of lignin comprises: acidifying a black liquor stream to a pH above 10 to form a dense liquid- lignin phase; isolating the dense liquid-lignin phase from the black liquor stream, wherein lignin within the dense liquid-lignin phase has a first average molecular weight; and exposing the lignin to heat and pressure for a period of time to reduce an average molecular weight of the lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight.

In another embodiment, a system for reducing molecular weight of lignin comprises: a source of black liquor; a pump in fluid communication with the source of black liquor; a carbonation column configured to countercurrently feed carbon dioxide into the carbonation column and adjusting the pH of the black liquor above about 9 to about 10 and isolate dense liquid-lignin phase from the black liquor, wherein the dense liquid-lignin phase comprises lignin having a first average molecular weight; a first reactor configured to heat the dense liquid-lignin phase at a pressure and for a residence time effective to reduce the first average molecular weight of lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight; and a second reactor in fluid communication with an acid source that is configured to reduce a pH of the dense liquid lignin to less than 4.

In yet another embodiment, a system for reducing molecular weight of lignin comprises: a source of black liquor; a pump in fluid communication with the source of black liquor; a carbonation column configured to countercurrently feed carbon dioxide into the carbonation column and reduce a pH of the black liquor to a range of about 11 to about 12 and isolate a first dense liquid-lignin phase from the black liquor, wherein the dense liquid-lignin phase comprises lignin having a first average molecular weight; a first reactor configured to heat the first dense liquid lignin phase at a pressure and for a residence time effective to reduce the first average molecular weight of lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight; a second carbonation column wherein carbon dioxide is fed to countercurrently contact the partially carbonated black liquor stream from the first carbonation column to adjust the pH of the stream to above about 9 to about 10 to isolate a third dense liquid-lignin phase where in the third dense liquid-lignin phase comprises lignin having a third average molecular weight; and a second reactor in fluid communication with an acid source that is configured to reduce a pH of the combined second and third dense liquid lignin phases to less than 4 or optionally a second reactor in fluid communication with an acid source that is configured to reduce a pH of the second dense liquid lignin phase to less than 4 and a third reactor in fluid communication with an acid source that is configured to reduce a pH of the third dense liquid lignin phase to less than 4.

Optionally, the system may further include an oxidation unit configured to introduce an oxidizing agent into the system and react the oxidizing agent with impurities within the black liquor or dense liquid-lignin phase in an amount effective to eliminate or substantially reduce the odor of the resulting lignin product having the second average molecular weight.

The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the Figures wherein the like elements are numbered alike:

FIG. 1 illustrates an example of the process for producing lignin disclosed in U.S. Pat. Pub. No. 2011/0294991;

FIG. 2 illustrates an exemplary process flow and system for reducing the molecular weight of liquid-lignin in accordance with the present invention;

FIG. 2 b illustrates an exemplary process flow and system for reducing the molecular weight of liquid-lignin showing the option of adding a strong base to adjust the pH;

FIG. 3 illustrates a process flow and system including an optional oxidation step added prior to carbonation in accordance with another embodiment of the present disclosure;

FIG. 4 illustrates a process flow and system including an optional oxidation step added after carbonation in accordance with another embodiment of the present disclosure;

FIG. 5 illustrates a process flow and system including an optional oxidation step added following the heating step; and

FIG. 6 illustrates a process flow and system including an optional oxidation step of the lignin product following solid-liquid separation in accordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are processes and systems for selectively reducing the molecular weight and/or molecular weight distribution of liquid lignin. The processes and systems are generally configured to first separate liquid-lignin such as by a carbonation process, or by an acid addition process, and heating the separated liquid lignin at an elevated temperature for a predetermined period of time and pressure effective to reduce an average molecular weight of the lignin as well as affect the molecular weight distribution of the lignin contained within the liquid-lignin. If necessary, sodium hydroxide (NaOH) or another strong base can be added to the liquid-lignin to raise its pH and further catalyze the reaction. This additional sodium can be recovered as sodium sulfate and returned to the host papermaking mill.

Advantageously, reducing the average molecular weight and/or molecular weight distribution by first separating the liquid-lignin phase from black liquor followed by heating overcomes many of the problems associated with the prior art. For example, by separating the liquid-lignin beforehand, the non-lignin components typically contained within the black liquor are not subjected to the time-temperature history employed to reduce the molecular weight of the lignin to the desired amount. Thus, avoiding thermal degradation of non-lignin components such as sugars and other labile components is a clear advantage of this invention. Moreover, the liquid-lignin phase comprises only about 20% of the total mass of the original black liquor, which offers the clear advantage of utilizing a smaller reactor system. Additionally, the liquid-lignin already has an elevated temperature since the heat of reaction of the carbon dioxide is retained within the liquid-lignin, elevating its temperature above the incoming black liquor by typically more than 10° C. Thus a process treating the liquid-lignin has much better energy efficiency than the prior art which requires elevating the temperature of the entire black liquor phase.

For ease in understanding, reference will now be made to carbonation processes. However, it should be apparent that an acid addition process can be employed to form and separate the dense liquid-lignin phase. Acid addition processes generally include adjusting the pH to effect phase separation of the dense liquid-lignin, which can then be processed as described in greater detail below with reference to the carbonation process to effect molecular weight reduction of the lignin.

In a carbonation process, lignin begins to precipitate immediately near the black liquor entrance and near the top of the column as the pH begins to be reduced by carbon dioxide (CO₂). As the pH decreases more and more lignin becomes insoluble and coalesces within column as liquid-lignin droplets, which settle rapidly from the residual carbonated black liquor under simple gravitational force. Countercurrently contacting the incoming black liquor with CO₂ creates a pH gradient in the column so that liquid-lignin droplets are created near the top that sweep and collect other liquid-lignin droplets that are forming at the lower pH in the lower zone of the column. The liquid-lignin particles have a natural affinity for other liquid-lignin particles, facilitating coalescence as they fall within the column. As the liquid-lignin particles fall through the column, the liquid-lignin particles collect other particles that are forming at the lower pH within the lower zones of the column. The dense particles then coalesce into a bulk liquid-lignin phase which accumulates at the bottom of the column. The melt point of the liquid lignin phase depends strongly on the concentration of cations (mainly sodium), the source of the lignin (the species of trees being pulped), and the level of water in the phase, hence its viscosity is difficult to predict.

The lower-density lignin-depleted phase, containing most of the sugars and valuable pulping chemicals, is returned to the recovery process of the host paper mill at a temperature higher than the temperature of the black liquor received. This eliminates the loss of energy as a major impediment for commercial implementation of lignin recovery by pulp mills, another clear advantage of the present invention. A carbonation process, such as the one disclosed in FIG. 1, employs a temperature profile within a range of about 90° C. to about 150° C. Practice of this invention to lower molecular weight of liquid-lignin requires heating to a higher temperature. In one embodiment, the temperature profile in the present process is within a range of about 150° C. to 300° C. and in another embodiment, the temperature profile is within a range of 150° C. to 200° C. In still other embodiments, the temperature profile in the present process is within a range of about 150° C. to about 190° C.; and in yet other embodiments, the temperature profile in the present process is within a range of about 160° C. to about 170° C.

As the temperature and/or pressure increases, the length of time to achieve the desired lower molecular weight decreases. Depending on the selected temperature, pressure, and time profile and the desired molecular weight and molecular weight distribution, the isolated liquid-lignin is heated for a period of time of about 1 minute to about 360 minutes, in other embodiments, from about 1 minute to 80 minutes, and in still other embodiments, from 1 minute to about 30 minutes. Molecular weight reduction may be accelerated by further increasing pH of the liquid lignin. In view of the foregoing, the carbonation processes generally include as a first step, pressurizing black liquor to between 50 and 3200 psig.

Methods and equipment for performing the heating step are well known to those skilled in the art. By way of example, a plug flow reactor may be employed to provide the desired time, temperature, and pressure profile to effect the molecular weight reduction. Other reactor configurations, including but not limited to batch stirred tank reactor, continuous stirred tank reactor (CSTR), ebullated bed reactor, and trickle bed reactor, will be obvious to one skilled in the art. Because of the high reactivity of lignin toward solids formation it is important to achieve good temperature control in the heating unit, especially avoiding hot spots, areas where the temperature is more than about 10° C. above the set temperature to minimize reactor fouling and plugging. Preferred methods for heating include heat transfer fluid (hot oil), electric resistance heating, and the like. Direct steam injection is another method which eliminates heat-transfer surfaces and the associated fouling issues and is compatible with high viscosity of the liquid lignin. An in-line mixer following the heating unit or the point of steam injection will facilitate more even temperature distribution within the viscous liquid-lignin phase. After heating, the dense lignin-rich phase can then be cooled and/or acidified to precipitate lignin with the desired molecular weight and desired molecular weight distribution.

A strong base could be added to the liquid-lignin, increasing its pH and catalyzing the molecular weight reduction. Sodium hydroxide is preferred as a strong base, since the sodium would be captured in the acid brine from the downstream lignin recovery process and returned to the host mill's recovery cycle where the sodium is recovered and used in NaOH and NaSH. Sufficient sodium hydroxide must be added to overcome the buffering effect of the sodium bicarbonate contained in the liquid-lignin. This may approach or exceed pH 12, starting with the carbonated liquid-lignin stream at pH 9-10.

As an optional step, sufficient oxidant may be reacted with the black liquor to mitigate and/or eliminate odors and species like sodium hydrosulfide (NaSH) that can evolve toxic gas (e.g., H₂S) if contacted with an acid much stronger than CO₂. Alternatively, oxidation of liquid lignin after separation offers the clear advantage of treating a smaller volume, about 20% of the total black liquor volume, so requires smaller, less costly equipment, and requires less oxygen since it avoids unwanted oxidation of sugars and other components of the carbonated black liquor.

Once the liquid lignin is separated, molecular weight and molecular weight distribution of the liquid lignin can be determined using size exclusion chromatography or other methods known to those skilled in the art. Similarly, after the heat treatment described above, molecular weight and molecular weight distribution of the treated liquid lignin can be determined. Likewise, after the lignin or lignin fractions have been recovered by acidification and separation, the molecular weight and molecular weight distribution of each material can be determined.

After the desired degree of molecular weight reduction is effected, the dense liquid-lignin is further acidified to a pH less than 4 in some embodiments, and to a pH of about 1.5 to about 3.5 in other embodiments to form solid liquid lignin. The acidification step can be performed by adding an acid (e.g., sulfuric acid). The particular acid is not intended to be limited. For instance, organic acids such as formic or acetic acid could be used. Protic acids, such as sulfuric acid, are favorable since their cost is low and because the sulfur can often be used in the host pulp mill to offset the normal sulfur make-up used by the mill to replace sulfur losses in the mill system, which produces internally the sodium hydrosulfide used as a pulping catalyst. For example, the dense liquid-lignin phase may be fed directly into another pressurized reactor where the stream is mixed with sulfuric acid. Depending on the nature of the lignin and the temperature of the reactor, the lignin forms either another dense liquid lignin phase or heavy solid granules that separate by settling. Either of these lignin forms can be pumped or discharged through a pressure-reducing valve into a countercurrent water extraction system, where residual acid and salt are removed, creating a low-ash lignin.

Turning now to FIG. 1, black liquor 12 is pressurized and fed through line 14 to pump 16, which feeds the pressurized stream through line 18 into the upper region of carbonation column 30. Carbon dioxide (CO₂, 31) is fed into column 30 and flows upward countercurrent to the downward flowing black liquor, while the pH is lowered to a pH of about 9-10 and entrained acid gases flow out of column 30 with any excess CO₂ through vent 32 to an effective scrubbing system, typically using strongly basic white liquor to remove toxic and malodorous gases. The lower end of column 30 is enlarged to form a settler 34 where the less dense lignin-depleted liquid rises to the top and exits through line 36, while the more dense liquid lignin exits through line 38 into the acidification vessel 70. Where acid 72 is added and the resulting slurry moves through line 74 into the solid-liquid separation unit 80, lignin. Lignin product 100 exits the solid liquid separation unit either as solid or as slurry through conduit 82, the precise nature of which will depend on the nature of the product stream, typically a pipe for slurry or a bin or conveyor for solid lignin.

Turning now to FIG. 2, there is shown a schematic diagram of an embodiment of an exemplary system of the present invention showing the steps, from a lignin containing stream, of carbonating to form a liquid-lignin, reducing the molecular weight of the liquid lignin, acidification and solid-liquid separation to recover lignin product. In this case, a heating unit 50 is inserted between settler 34 and acidification unit 70. The final lignin product will be suitable for applications that are generally insensitive to the odor of the final product, as typically would be the case when the lignin is to be used as a fuel or as a binder for energy pellets.

Also shown in FIG. 2 b is the optional addition of a strong base 46, typically NaOH, to adjust pH of the stream. The strong base 54 is transported through line 56 into the liquid lignin in line 38.

Turning now to FIG. 3, there is shown a schematic diagram of an embodiment of an exemplary system of the present invention showing the optional step of reacting the black liquor stream with an oxidant, e.g., air, oxygen, peroxide or the like, prior to carbonation. In this case, oxidation unit 20 is inserted between pump 16 and carbonation column 30. Pressurized black liquor is fed into oxidation unit 20 through line 16, oxidant is fed through line 22 and the oxidized liquid is fed into carbonation column 30 through line 24. Exemplary equipment for this reaction is a Hydrodynamics Shockwave Power Reactor®, shown at 20 in FIG. 3. Oxidation also has a substantial heat of reaction, raising the temperature of the stream, typically about 50° C. depending on the reactants within the aqueous stream and its solids content.

Turning now to FIG. 4, there is shown a schematic diagram of another embodiment of an exemplary system of the present invention showing the optional step of oxidizing the liquid lignin stream after carbonation. This option offers the advantage of not oxidizing the entire black liquor stream, so equipment can be smaller and less oxidant is needed. In this case, oxidation unit 40 is inserted between settler 36 and heating unit 50. Liquid lignin is fed into oxidation unit 40 through line 38. Oxidant is fed through line 42 and the oxidized liquid is fed into heating unit 50 through line 44. Depending on the heat of reaction with the oxidant with the liquid lignin, heating unit 50 may not be required due to the inherent temperature increase.

Turning now to FIG. 5, there is shown a schematic diagram of an embodiment of an exemplary system of the present invention showing the optional step of oxidizing the liquid lignin stream after heating to lower molecular weight. In this case, oxidation unit 60 is inserted between heating unit 50 and acidification unit 70. This option may be advantageous when heating produced especially reactive species that complicate downstream operations. Heated, and optionally cooled, liquid-lignin is fed into oxidation unit 60 through line 52. Oxidant is fed through line 62 and the oxidized liquid is fed into acidification unit 70 through line 64.

Turning now to FIG. 6, there is shown a schematic diagram of an embodiment of an exemplary system of the present invention showing the optional step of oxidizing the liquid lignin stream after heating to lower molecular weight. In this case, oxidation unit 90 is located after solid-liquid separation unit 80. This option may be advantageous when earlier oxidation produces species, surfactants for example, that adversely affect downstream operations. Solid lignin, optionally as slurry, is fed into oxidation unit 90 through line 82, oxidant is fed through line 92 and the oxidized lignin product exits through line 100.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A process for reducing molecular weight of lignin comprising: carbonating a black liquor stream at a temperature between about 80° C. and 200° C. and adjusting the pH above 10; recovering a dense liquid-lignin phase, wherein lignin within the dense liquid-lignin phase has a first average molecular weight; and exposing said dense liquid-lignin to heat and pressure for a predetermined period of time to reduce an average molecular weight of said dense liquid-lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight.
 2. The process according to claim 1 further comprising, acidifying the dense liquid lignin with an acid to a pH between 1.5 and 3.5.
 3. The process according to claim 1, wherein said carbonating of the black liquor is effective to adjust the pH to between 11 and
 13. 4. The process according to claim 1, wherein exposing said dense liquid-lignin to heat and pressure is at a temperature of about 150° C. to about 300° C.
 5. The process according to claim 1, wherein exposing the dense liquid lignin to heat and pressure is at a temperature of about 150° C. to about 190° C.
 6. The process according to claim 1 where a strong base is added to the dense liquid-lignin to raise its pH and catalyze lowering of the average molecular weight.
 7. The process according to claim 1, further comprising adding an oxidizing agent to said black liquor prior to carbonating in an amount effective to eliminate or substantially reduce the odor of the resulting lignin product.
 8. The process according to claim 7, wherein said oxidizing agent is a member of the group consisting of oxygen, air, a peroxide, or mixtures thereof.
 9. The process according to claim 1, wherein exposing said dense liquid-lignin to heat and pressure for the period of time to reduce the average molecular weight of the lignin to a second average molecular weight comprises introducing the dense liquid-lignin into a plug-flow reactor.
 10. The process according to claim 1, wherein said black liquor has a solids content between 10 to 70 weight percent.
 11. A process for reducing molecular weight of lignin, comprising: acidifying a black liquor stream to a pH above 10 to form a dense liquid-lignin phase; isolating the dense liquid-lignin phase from the black liquor stream, wherein lignin within the dense liquid-lignin phase has a first average molecular weight; and exposing said lignin to heat and pressure at a temperature of about 150° C. to about 250° C. for a period of time to reduce an average molecular weight of the lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight.
 12. The process according to claim 11, wherein subsequent to exposing the dense liquid-lignin to heat and pressure for the period of time to reduce the average molecular weight of the lignin to the second average molecular weight acidifying the dense liquid-lignin with an acid to a pH less than
 4. 13. The process according to claim 11 wherein a strong base is added to the dense liquid-lignin to raise its pH and catalyze lowering the average molecular weight.
 14. The process according to claim 11, further comprising adding an oxidizing agent to the black liquor prior to acidifying the black liquor in an amount effective to eliminate or substantially reduce the odor of the resulting lignin product.
 15. The process according to claim 11, wherein said oxidizing agent is a member of the group consisting of oxygen, a peroxide, or mixtures thereof.
 16. The process according to claim 11, wherein said black liquor has a solids content between 10 to 70 weight percent.
 17. A process for reducing molecular weight of lignin comprising: carbonating a black liquor stream at a temperature between about 80° C. and 200° C. and adjusting the pH above 10; recovering a first dense liquid-lignin phase, wherein lignin within the first dense liquid-lignin phase has a first average molecular weight; exposing the first dense liquid-lignin to heat and pressure for a pre-determined period of time to reduce a first average molecular weight of the lignin to a second average molecular weight, wherein the second average molecular weight is less than the first average molecular weight, further carbonating the less dense liquid stream isolated from the first carbonation to lower a pH to a range of about 9 to about 10 recovering a second dense liquid-lignin phase.
 18. The process according to claim 17 further comprising, acidifying the dense liquid-lignins, either combined or separately, with an acid to a pH less than
 6. 19. The process according to claim 17, further comprising, acidifying the dense liquid-lignins, either combined or separately, with an acid to a pH between 1.5 and 3.5.
 20. The process according to claim 17, wherein said carbonating of said black liquor is carried out by contacting said black liquor with carbon dioxide countercurrently.
 21. The process according to claim 17, wherein said carbonating of the black liquor is effective to reduce the pH to between 9.0 and 10.5.
 22. The process according to claim 17, wherein exposing the dense liquid lignin to heat and pressure is at a temperature of about 150° C. to about 300° C.
 23. The process according to claim 17, wherein exposing the dense liquid-lignin to heat and pressure is at a temperature of about 150° C. to about 190° C.
 24. The process according to claim 17 where a strong base is added to the dense liquid lignin to raise its pH and catalyze lowering of the average molecular weight.
 25. The process according to claim 17 further comprising, adding an oxidizing agent to the black liquor prior to carbonating the black liquor in an amount effective to eliminate or substantially reduce the odor of the resulting lignin product.
 26. The process according to claim 17, wherein the oxidizing agent is a member of the group consisting of oxygen, air, a peroxide, and mixtures thereof.
 27. The process according to claim 17, wherein exposing the dense liquid- lignin to heat and pressure for the period of time to reduce the average molecular weight of the lignin to a second average molecular weight comprises introducing the dense liquid lignin into a plug-flow reactor.
 28. The process according to claim 17, wherein said black liquor has a solids content between 10 to 70 weight percent.
 29. A process comprising the steps of pH fractionation, using controlled acidification of black liquor to separate a first liquid-lignin fraction of relatively high molecular weight, further acidification of produce a second liquid-lignin fraction of lower molecular weight, heating the first liquid-lignin fraction to lower its molecular weight, and blending the resulting two fractions to produce a blend of desirably lower molecular weight.
 30. A process according to claim 29, where said black liquor feedstock is oxidized.
 31. The process according to claim 30, wherein said oxidizing agent is a member of the group consisting of comprises oxygen, air, a peroxide, and mixtures thereof. 